Etymology

From Middle Dutch selsiene, selsane, selsene, also selsame, from Old Dutch *seldsiuni, *seldsāni, from Proto-Germanic *seldasiuniz ‎(“rarely seen”), a compound of *selda- ‎(“rare”) + *siuniz ‎(“sight”). The formation with -zaam is by analogy and occurred in Middle Dutch because the original formation of the word was no longer understood. The same happened in for example German seltsam and Middle Low German seltsam (from which Swedish sällsam).

Pronunciation